Conventional low-coherent interferometry can only perform well when it is used under the help of good anti-vibration devices so that it can use an interferometric scanning technique to find different groups of surface points respectively on different vertical scanning levels to precisely measure the surface profile of a static object. However, although using anti-vibration devices is very efficient in reducing the vibration amplitude of a measured object affected by environmental disturbance such as structure vibration, air turbulence or acoustic instability, it still remains difficult to make the measured object completely static. As such interferometric scanning is often performed in site for production lines to fabricate precision parts, such as micro-electro-mechanical system (MEMS) components, IC wafers, or LCD panels, and the performance of such interferometric scanning is inevitably affected by environmental vibrations. Thus, an improved low-coherent interferometry using interferometric scanning techniques to measure the surface profile of an object suffering environmental disturbance is prominently needed.
Please refer to FIG. 1, which shows the way a conventional low-coherent interferometry is performed. In FIG. 1, an interferometric objective 11 contains a beam splitter 111, by which a downward beam incident into the interferometric objective 11 is divided into a downward transmissive beam and a upward reflected beam. Thereafter, the upward reflected beam is reflected back to the beam splitter 111 to form a reference beam by a reflective layer 113 coated at a small area on a transparent substrate 112 in the interferometric objective 11 while the downward transmissive beam is projected on the surface of a measured object 12 and is scattered to form a scattered field. A part of the scattered field is projected back to the interferometric objective 11 to form an object beam and is combined with the reference beam by the beam splitter 111 to form an upward interference field used for forming an interference pattern. When the light used to form an interference pattern is a high-coherent one, all local areas in the interference pattern can be clear (focused and with a reasonable contrast). However, when the light used to form an interference pattern is a low-coherent one, only some local areas in the interference pattern can be clear and other local areas can't. Moreover, when the reference beam is a plane wave, all the object surface points on a flat plane 13 make their corresponding local areas in the object beam have almost the same optical path as the reference beam after the reference beam and the object beam are combined by the beam splitter 111. Therefore, all interference fringes in the low-coherence interference pattern disclose that all the object surface points corresponding to these fringes are on the plane 13.
When a conventional low-coherent interferometric scanning technique is used for measuring the surface profile of a static object 12, the distance s between the datum plane 15 of the interferometric objective 11 and the datum plane 16 of the measured object 12 can be easily controlled to be a specified value. It noted that the distance L between the datum plane 15 and the plane 13 is constant. When the interferometric objective 11 goes upward by a distance zu (i.e. the datum plane 15 goes upward by zu), the plane 13 also goes upward by zu. When the interferometric objective 11 goes downward by a distance zd, the plane 13 goes downward by zd as well. Therefore, by changing the position of the interferometric objective 11 (scanning), the plane 13 can in turn intersect with different levels on the surface of the measured object 12 to form corresponding interference patterns. Because the differences for the distances s for forming all the interference patterns may be known in advance, the surface profile of the measured object 12 can be derived by analyzing the interferences on the interference patterns.
When a conventional low-coherent interferometric scanning technique is used to measure the surface profile of an object while the measured object is disturbed by vibrations or other environmental disturbances, the distance s(t) between the datum plane 15 of the interferometric lens and the datum plane 16 of the measured object 12 can't be easily controlled to be a specified value because it depends on time t. By changing the position of the interferometric objective 11 (i.e. performing a scanning operation), the plane 13 can in turn intersect with different levels on the surface of the measured object 12 to form interference patterns. Because the setup is disturbed by vibrations, the distance s(t) for forming every interference pattern becomes time-variant and unpredictable. The surface profile of the measured object 12 can't be derived by analyzing the interferences on the interference patterns until the distance s(t) for forming every interference pattern is measured or the differences for the distances s(t) for forming all the interference patterns become known. For deriving the distance s(t), it can be obtained by directly measuring the distance between the datum plane 15 of the interferometric objective 11 and the datum plane 16 of the measured object 12, or it can be obtained by the following process: obtaining a distance a(t) between an environment datum level 14 and the datum plane 15; obtaining another distance b(t) between the environment datum level 14 and the datum plane 16 of the measured object; and then obtaining the distance variation s(t) by the formula: s(t)=b(t)−a(t). Similarly, the difference of two distances s(t) can be derived by subtracting the difference of two distances a(t) from the difference of two distances b(t).
There are four U.S. patents (Pub. No. 1996/5589938, Pub. No. 1999/5999263, Pub. No. 2003/6624894, and Pub. No. 2005/0237535) that disclose related methods to measure the differences between the distances s(t) for forming all the interference patterns. All the methods detect the actual displacement of the measured object or the interferometric objective to derive the changes of s(t). Therefore, all the above-mentioned published patents measure the instant distance s(t) for forming each of interference patterns and the interference patterns are formed from non-uniform increments (caused by vibrations) for the distance s(t). However, the distance s(t) for forming each of interference patterns can be fixed to be a specified value s that is not affected by vibrations and the interference patterns can be formed from uniform increments (achieved by vibration-resistant capability) for the distance s when the method proposed in this patent is used.